Flat-shaped batteries, small in size and thickness, are widely used as power source for devices whose miniaturization is desired, such as watches and remote controllers for automotive keyless entry system. They are also widely used as power source for applications where long-term continuous supply of electric power is required, such as memory backup for office automation (OA) or factory automation (FA) equipment. They are also used as power source for various meters and measurement apparatus, and their applications are expanding. In recent years, there is a growing demand for them to be used in severe environments, for example, in automobiles parked outdoors in cold districts or under the hot summer sun, or industrial machines used in high temperature environments, and improvement is underway for their higher performance.
Flat-shaped batteries include a flat case serving as a positive terminal. Positive and negative electrodes and a separator interposed therebetween are housed in the case, and an electrolyte is poured into the case. The opening of the case is closed with a sealing plate serving as a negative terminal, with a gasket interposed therebetween. The positive electrode is, for example, a circular plate-shaped pellet of a positive electrode material mixture including an active material such as manganese dioxide or fluorinated graphite. The positive electrode pellet is housed in the case such that one bottom surface thereof faces the bottom of the case.
To improve the current collecting ability of the positive electrode, a plate-shaped current collector is disposed between the positive electrode pellet and the bottom of the case. The current collector, at one side thereof, is in contact with the positive electrode pellet, and at the other side thereof, for example, is spot-welded to a center portion of the bottom of the case, providing electrical connection between the positive electrode pellet and the case.
Disadvantageously, the battery internal pressure increases when the battery is placed in high temperatures or other extreme conditions. The increase in battery internal pressure may cause the case to “swell” such that the bottom of the case bulges outward. If the case swells as above, the current collector may deform as well to follow the deformation of the case, and the contact pressure between the positive electrode pellet and the current collector may drop. As a result, the internal resistance of the battery increases, and the discharge characteristics of the battery degrade.
To solve the above problem, Patent Literature 1 discloses forming a plate-shaped current collector such that the cross section at its center is U-shaped. This can impart a spring function to the current collector, and thus can improve the contact between the positive electrode and the current collector.